Window assemblies in residential, commercial and industrial buildings may include one or more window sashes that are movable vertically within a window jamb. Window sashes that move vertically to open and close often include two or more balance assemblies. The balance assemblies urge the window sash upward (i.e., toward an open position for a lower sash or toward a closed position for an upper sash) to assist a user in moving the window sash and to retain the window sash at a position selected by the user.
Locking mechanisms to lock the carrier (also known as a “shoe”) in the jamb channels when the sash of a tilt-sash window assembly is tilted are known in the art. U.S. Pat. No. 5,353,548, entitled “CURL SPRING SHOE BASED WINDOW BALANCE SYSTEM”, issued Oct. 11, 1994 to Westfall, discloses a window balance system for a tilt-sash window assembly having a pair of constant force curl springs having curled convolutions carried by sash shoes and free end regions mounted in sash shoe channels above the region of travel of the shoes. The curl tendency of the springs imparts a lift to the curled spring convolutions, and the shoes transmit the lift to the sash. The springs curl into the convolutions as the shoes rise, and the springs uncurl from the shoes into the shoe channels when the shoes move downward. An annular cam on a receiver in the shoe locks the shoe in the shoe channel when the sash is tilted outward. The receiver has an opening which receives a pin or pivot bar connected to the sash such that when the sash is tilted, the receiver rotates with the sash, whereby the cam rotates to separate the two body parts of the shoe body such that they bind against the shoe channel to prevent upward or downward movement of the shoe while the sash is tilted. The disclosure of U.S. Pat. No. 5,353,548 is hereby incorporated herein in its entirety.